The present invention relates to a short arc mercury lamp and a lamp unit. In particular, the present invention relates to a short arc mercury lamp and a lamp unit used as a light source for an image projection apparatus such as a liquid crystal projector and a digital micromirror device (DMD) projector.
In recent years, an image projection apparatus such as a liquid crystal projector or a projector using a DMD has been widely used as a system for realizing large-scale screen images, and a high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. In the image projection apparatus, light is required to be concentrated on a very small area of a liquid crystal panel or the like, so that in addition to high intensity, it is also necessary to achieve a light source close to a point light source.
As high pressure discharge lamps that can meet this need, the research and development of metal halide lamps was conducted first of all. However, it was found that when the arc length was reduced to achieve a light source close to a point light and high intensities, the arc width is increased in the case of metal halide lamps. Therefore, nowadays, a short arc ultra high pressure mercury lamp that is closer to a point light and has a high intensity has been noted widely as a promising light source. In the ultra high pressure mercury lamp, 90% of the entire luminous flux emit light in an effective region, whereas in the metal halide lamps having a large arc width, only 50% of the entire luminous flux emit light in an effective region. This occurs for the following reasons. In the case of the metal halide lamps, the average excitement potential of the enclosed metal is comparatively as low as 4 to 5 eV, and therefore emission occurs in the vicinity of the arc so that the arc width is large. On the other hand, in the case of the ultra high pressure mercury lamps, since mercury has a higher average excitement potential (7.8 ev) than that of the enclosed metal for the metal halide lamp, emission occurs in the central region of the arc, and thus the arc width is small. Therefore, the average intensity of the arc in the ultra high pressure mercury lamp can be higher than that of the metal halide lamp.
Referring to FIGS. 12A and 12B, a conventional short arc ultra high pressure mercury lamp 1000 will be described.
FIG. 12A is a schematic view of an ultra high pressure mercury lamp 1000. The lamp 1000 includes a substantially spherical luminous bulb 110 made of quartz glass, and a pair of sealing portions (seal portions) 120 and 120xe2x80x2 also made of quartz glass and connected to the luminous bulb 110. A discharge space 115 is inside the luminous bulb 110. A mercury 118 in an amount of the enclosed mercury of, for example, 150 to 250 mg/cm3 as a luminous material, a rare gas (e.g., argon with several tens kPa) and a small amount of halogen are enclosed in the discharge space 115.
A pair of tungsten electrodes (W electrode) 112 and 112xe2x80x2 are opposed with a certain distance D (e.g., about 1.5 mm) in the discharge space 115. Each of the W electrodes 112 and 112xe2x80x2 includes an electrode axis (W rod) 116 and a coil 114 wound around the head of the electrode axis 116. The coil 114 has a function to reduce the temperature at the head of the electrode. The respective electrode axes 116 of the W electrodes 112 and 112xe2x80x2 are matched to be on the same axis to maintain the optical symmetry, and therefore, the electrode central axes 119 of the W electrodes 112 and 112xe2x80x2 are matched to each other.
The electrode axis 116 of the W electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, and the W electrode 112 and the Mo foil 124 are electrically connected by a welded portion 117 where the electrode axis 116 and the Mo foil 124 are welded. The sealing portion 120 includes a glass portion 122 extended from the luminous bulb 110 and the Mo foil 124. The glass portion 122 and the Mo foil 124 are attached tightly so that the airtightness in the discharge space 115 in the luminous bulb 110 is maintained. In other words, the sealing portion 120 is sealed by attaching the Mo foil 124 and the glass portion 122 tightly for foil-sealing. The sealing portions 120 have a circular cross section, and the rectangular Mo foil 124 is disposed in the center of the inside of the sealing portion 120. The Mo foil 124 of the sealing portion 120 includes an external lead (Mo rod) 130 made of molybdenum on the side opposite to the side on which the welded portion 117 is positioned. The Mo foil 124 and the external lead 130 are welded with each other so that the Mo foil 124 and the external lead 130 are electrically connected at a welded portion 132. The structures of the W electrode 112xe2x80x2 and sealing portion 120xe2x80x2 are the same as those of the W electrode 112 and sealing 120, so that description thereof will be omitted.
As shown in FIG. 12B, the lamp 1000 is electrically connected to a ballast 1200 for lighting. When the ballast 1200 is operated in the state where the external lead 130 is connected to the ballast 1200, the lamp 1000 turns on.
Next, the operational principle of the lamp 1000 will be described. When a start voltage is applied to the W electrodes 112 and 112xe2x80x2 from the ballast 1200 via the external leads 130 and the Mo foils 124, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space 115 of the luminous bulb 110, and thus the mercury 118 is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes 112 and 112xe2x80x2. The higher the mercury vapor pressure of the lamp 1000 is, the higher the emission efficiency is, so that the higher mercury vapor pressure is suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 25 MPa.
The conventional lamp 1000 sometimes failed to turn on when the lamp was turned on again after turning off, although the lamp was used properly. The cause of the failure of lamp lighting was conventionally not clear. However, as a result of in-depth research, the inventors of the present invention found that this was caused by the fact that, as shown in FIG. 13, a bridge (mercury bridge) 140 of mercury 118 occurs between the W electrodes 112 and 112xe2x80x2, so that the W electrodes 112 and 112xe2x80x2 are short-circuited.
When a start voltage is applied to the lamp 1000 in a state where the electrodes are short-circuited by the mercury bridge 140, a large amount of current flows in the lamp 1000. As a result, the ballast 1200 detects operation abnormality and automatically stops the start of the lamp lighting. After the start of the lamp lighting is stopped, the mercury bridge 140 still remains, so that the lamp 1000 is not turned on, even if the ballast 1200 starts operating for lighting again.
It seems that the mercury bridge 140 is formed in the following manner. When turning on the lamp 1000, the temperature at the W electrodes 112 and 112xe2x80x2 causing discharge is about 3000xc2x0 C., and the temperature at the luminous bulb 110 positioned around the W electrodes is about 1000xc2x0 C. When the lamp 1000 is turned off, the W electrode 112 made of a metal is cooled faster than the luminous bulb 110 made of glass. Therefore, mercury vapor in the discharge space 115 is condensed more on the W electrode 112 than on the inner wall of the luminous bulb 110, so that the mercury vapor is likely to precipitate as a mercury ball (Hg ball) in the W electrode 112.
When the W electrode 112 is cooled and the condensation of the mercury vapor proceeds, as shown in FIG. 14A, the Hg ball 118 is grown concentrically from the head 111 of the W electrode 112 towards the head of the opposing W electrode. Since the surface tension is applied to the Hg ball 118, the growth direction of the Hg balls 118 is the same direction as that of the electrode central axis 119. When the growth of the Hg ball 118a of the W electrode 112 proceeds and becomes in contact with the Hg ball 118b grown from the W electrode 112xe2x80x2, the two Hg balls are integrated into one ball by the surface tension, so that as shown in FIG. 14B, the mercury bridge 140 is formed. Once the mercury bridge 140 is formed, the W electrodes 112 and 112xe2x80x2 are short-circuited, and the start voltage cannot be applied normally to the lamp 1000, resulting in the failure of the operation of the lamp 1000.
Compared with a lamp having a comparatively long (e.g., about 1 cm) distance (electrode arrangement distance) D between the W electrodes 112 and 112xe2x80x2, in the case of the lamp 1000 having a short arc with a distance D of about 2 mm or less, the amount of mercury to be enclosed in the discharge space 115 is increased to suppress the current increase involved in achieving short arc. Therefore, in the case of the short arc lamp, in addition to a short distance D, the amount of mercury condensed in the W electrode 112 becomes large, so that the mercury bridge 140 is formed more easily than in lamps having a comparatively long distance D. The distance D tends to be short to meet the need of achieving higher intensities and a light source close to a point light source, and therefore the problem of the mercury bridge will become more serious.
Therefore, with the foregoing in mind, it is a main object of the present invention to provide a short arc mercury lamp having improved reliability of lamp operation in which the mercury bridge is prevented or suppressed from being formed.
A short arc mercury lamp of the present invention includes a luminous bulb enclosing at least mercury as a luminous material and a pair of electrodes opposed to each other; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively; wherein an electrode central axis of one of the pair of electrodes is dislocated from an electrode central axis of the other electrode of the pair of electrodes, and a shortest distance d (cm) between a head of one of the electrodes and a head of the other electrode is larger than a value of (6M/13.6xcfx80)⅓ when a total mass of the enclosed mercury is M (g).
Another short arc mercury lamp of the present invention includes a luminous bulb enclosing at least mercury as a luminous material and a pair of electrodes opposed to each other; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively, wherein an electrode central axis of one of the pair of electrodes and an electrode central axis of the other electrode are not on the same common axis, and a projection plane where a head plane of one of the electrodes is projected along a direction of the electrode central axis of the one of the electrodes is in contact with or at least partially overlapped with a head plane of the other electrode.
Still another short arc mercury lamp of the present invention includes a luminous bulb enclosing at least mercury as a luminous material and a pair of electrodes opposed to each other; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively; wherein a shortest distance d between the head of one of the electrodes and the head of the other electrode is longer than an arrangement distance D between one of the electrodes and the other electrode.
It is preferable that the shortest distance d (cm) between the head of one of the electrodes and the head of the other electrode is larger than a value of (6M/13.6xcfx80)⅓ when a total mass of the enclosed mercury is M (g).
In one embodiment of the present invention, lighting system is an alternating current lighting system.
A lamp unit of the present invention includes the above-described short arc mercury lamp and a reflecting mirror for reflecting light emitted from the mercury lamp.
A high pressure mercury lamp of the present invention includes a luminous bulb enclosing at least mercury as a luminous material and a pair of electrodes opposed to each other; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively; wherein an electrode central axis of one of the pair of electrodes is dislocated from an electrode central axis of the other electrode, and a shortest distance d (cm) between a head of one of the electrodes and a head of the Hi other electrode is larger than a value of (6M/13.6xcfx80)⅓ when a total mass of the enclosed mercury is M (g).
It is preferable that the arc length of the high pressure mercury lamp is 2 mm or less, and a total mass of the enclosed mercury is 150 mg/cm3 or more.
According to the short arc mercury lamp of the present invention, the electrode central axis of one electrode is dislocated from the electrode central axis of the other electrode. Therefore, even if mercury enclosed in a luminous bulb is condensed and is grown from the head of one electrode, the mercury does not become in contact with the mercury grown from the other electrode along the electrode central axis of the other electrode, compared with the prior art. As a result, the mercury bridge can be prevented or suppressed from being formed between the pair of electrodes. Furthermore, the electrode central axes are not matched with each other, so that even if the mercury bridge is formed, the surface tension is not applied to the formed mercury bridge symmetrically. Therefore, the mercury bridge cannot stay stably between the heads of the electrodes, and even if the mercury bridge is formed, the mercury bridge can be removed easily. Thus, the reliability of the lamp operation can be improved.
Furthermore, according to another short arc mercury lamp of the present invention, in addition to the prevention or suppression of the mercury bridge by the fact that the respective electrode central axes of the pair of electrodes are not on the same and common axis, the following advantage is provided. Since the projection plane of one electrode is in contact with the head plane of the other electrode, or at least a part is overlapped, this embodiment is substantially not different from the case where the axes of the electrodes are on the same axis, at least regarding the stability of discharge.
Furthermore, according to still another short arc mercury lamp of the present invention, the shortest distance d between the head of one electrode and the head of the other electrode is longer than the arrangement distance D between one electrode and the other electrode. Therefore, the mercury grown from the heads of the two electrodes are not in contact with each other, compared with the prior art, even if the arrangement distance D is the same as that of the prior art. As a result, the formation of the mercury bridge can be prevented or suppressed. Thus, the reliability of the lamp operation can be improved. Furthermore, since the arrangement distance D is the same, in the structure where the mercury lamp and a reflecting mirror are combined, the same light focusing efficiency as that of the conventional structure can be obtained.
According to the mercury lamp of the present invention, the formation of the mercury bridge can be prevented or suppressed, and therefore the reliability of the lamp operation can be improved. Furthermore, as a result of preventing or suppressing the formation of the mercury bridge, it is possible to increase the amount of enclosed mercury, so that the performance of the mercury lamp can be improved.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.